Raman light scattering has been successfully used in critical care situations to continuously monitor a patient's respiratory gases. This technique is based on the effect which occurs when monochromatic light interacts with vibrational/rotational modes of gas molecules to produce scattered light which is frequency shifted from that of the incident radiation by an amount corresponding to the vibrational/rotational energies of the scattering gas molecules. If the incident light photon loses energy in the collision, it is re-emitted as scattered light with lower energy and consequently lower frequency than the incident photon. In a similar manner, if the incident photon gains energy in the collision, it is re-emitted as scattered light with higher energy and higher frequency than the incident photon. Since these energy shifts are species-specific, analysis of the various frequency components present in the Raman scattering spectrum of a sample provides chemical identification of the gases present in the scattering volume. The intensity of the various frequency components or Raman spectral lines provides quantification of the gases present, providing suitable calibrations have been made. In this manner, Raman light scattering can be employed to determine the identity and quantity of various respiratory and anesthetic gases present in a patient's breath in operating room and intensive care situations.
In addition to critical care situations, Raman light scattering gas analysis can also be used in many industrial applications such as stack gas analysis for combustion control, process control, fermentation monitoring, and pipeline gas mixture control. This analysis technique can also be extended to meet environmental monitoring needs in many areas such as escaped anesthetic agents in the operating room, air pollution, auto emissions testing and submarine atmosphere monitoring.
Systems developed for analysis of gases in critical care situations utilizing Raman scattering typically employ gas cells which contain a sample of the patient's respiratory gas to be analyzed. The gas sampling cell is located either within the resonant cavity of a laser or outside the cavity. In an intracavity system, a laser beam is directed through the resonant cavity such that it intercepts the gas within the sampling cell. Raman scattered light from the gas analysis region within the cell is collected by a collection optic and directed through one or more interference filters. The collection optics and interference filters and possibly focusing optics in turn transmit the Raman scattered light to appropriate detectors for quantitating each specific Raman signal, and thus, each specific gas comprising the respiratory sample.
Windows are commonly provided on either end of the gas sampling cell to protect surrounding optical elements and filters from contaminants which may be present in the gas sample. The windows further serve to confine the gas sample within the chamber, minimizing the volume of the sample and thus improving response time. In some systems, the gas cell windows can be oriented at Brewster's angle to select and improve the transmission of a particular polarization of light passing through the sample. In this manner, optical losses in the laser beam which passes through the cell are minimized. However, the gas sample, in combination with particulates often carried with the sample, contaminates the cell windows and degrades the performance of the system. For example, this contamination may result in undesirable light scattering, and thus, the corresponding laser power may drop significantly. If untreated and uncorrected, the system will cease to function properly. Current respiratory gas analysis systems require periodic replacement or cleaning of the gas cell to compensate for the accumulation of contaminants. This is generally a time-consuming process which involves not only the replacement or cleaning of the cell, but also, recalibration of the system, both at substantial expense in both time and money.
An improved apparatus for confining a gas sample within an analysis region can be provided by removing the windows from the ends of the gas sampling cell and forming air dams or curtains of air between the sample gases and the optical elements or the surrounding optical elements. Such a system is described in commonly assigned copending application Ser. No. 522,533. These systems are quite adequate in applications where the index of refraction of the sample gases does not change. However, in applications where the index of refraction of the sample gases is variable, it is often difficult to maintain optimum laser power in the resonant cavity. This is because index of refraction differences can cause laser beam movement and alignment changes, which affect the optical characteristics of the resonant cavity as well as the detection optics. In cases where the changes in index of refraction are predictable or known, it is possible to compensate by an appropriate calibration procedure. However, in many applications these changes are not predictable or known. For example, in a respiratory gas analysis system, the index of refraction of the gases being drawn into the gas cell changes with each breath taken by the patient.
When the laser beam passes through the interfacial regions or interfaces P and P' (shown in FIG. 4) formed between the air dam buffer gas and the gas being analyzed in the gas cell, it is "steered" by that interfacial region between the gases to a greater or lesser extent (as shown in FIG. 4). The extent to which the beam is "steered" is dependent on at least two things: 1) the difference between the refractive index of the analyte gas in the analysis portion of the gas cell (n.sub.A) and the refractive index of the air dam buffer gas (n.sub.S); and 2) the angles formed by the intersection of the laser beam axis with the interfacial regions P and P'.
The composition, and thus the index of refraction (n.sub.S), of the air dam buffer gas does not normally change during use. However, the index of refraction (n.sub.A) of the analyte gas mixture inside the gas cell often changes as the makeup of that gas changes. For example, in medical applications, the index of refraction of the gas/agent mixture changes appreciably when the gas in the gas cell changes from simple room air to a mixture with a high concentration of Nitrous Oxide. Furthermore, if the gas/agent mixture comprises respiratory gases from a patient, the index of refraction of the sample gas changes as the patient inhales and exhales.
At least two significant problems can occur when these index of refraction changes/beam steering effects occur. First, when the gas composition changes, the index of refraction changes, and therefore the path of the laser beam through the resonant cavity is altered. When the beam path changes, it changes the location at which the beam reflects off the mirror at the other end of the resonator. When this alignment change occurs, the lasing efficiency can drop significantly, thus causing loss in laser power which in turn causes the intensity of the Raman scattered light going to the detectors to drop. This loss of Raman signal reduces the signal to noise ratio of the system and is therefore undesirable. Secondly, when the path of the laser beam through the gas analysis cell changes, it may cause the laser beam to move out of the location which optimizes the efficiency of the detector system. The Raman scattered light which is coming from the laser beam and being focused on the detectors is used to identify and quantify the analyte gases. A shift of the laser beam location relative to the detector system changes the amount of light falling on the detectors and therefore changes the measurements being made in unpredictable ways. The present invention dramatically reduces these undesirable effects caused by varying gas composition and fluctuations in the index of refraction of the gases in the gas analysis cell.
Another factor which has been found to affect the lasing efficiency and thus the Raman scattered light intensity, is the gas pressure within the portion of the cavity containing the buffer and/or sample gases. Typically, the optical elements are aligned to maximize the circulating laser light intensity within the sample chamber. However, it has been discovered that as the pressure of the gas along the path of the laser beam changes, the intensity of the laser beam fluctuates. While some previous analysis systems monitor the gas pressure, none has recognized the relationship between the pressure and the light beam intensity. One prior system, U.S. Pat. No. 4,784,486 to Van Wagenen, monitors the gas pressure in the sample chamber and signals if the pressure drops below a threshold value, usually an indication of a plugged filter or other obstruction in the flow path of the sample gas from the patient to the chamber. The present invention improves the overall performance of the instrument by maintaining a constant pressure in the gas chamber, thus maintaining optimum signal intensity.